94 TERTIARY INSECTS OF NORTH AMERIOA. 



Order THYS^NXJH^ Latreille. 



All we have hitherto known of fossil Thysanura has been derived from 

 inclusions in amber,^ of which about eig'hteen species of six or seven genera 

 of Lepismatidfe and ten species of four genera of Poduridfe are known ; 

 among them are some very remarkable forms. Florissant has yielded two 

 species of this group, the first that have been found in rock deposits, and 

 one of them in considerable numbers, representing a species of exceptional 

 interest. 



Suborder BALLOSTOMA Scudder. 



For characters see under the single species, at the end. 



PLANOCEPHALUS Scudder. 

 Planocephalus aselloides. 



(See figures iu text below.) 



Planocephalus aselloides ScHdd., Mem. Nat. Acad. Sciences, III, 85-90 Figs. (1885); in Zittel, Handb. 

 Palajont., I, ii, 772, Fig. 972 (1885) ; Bertk., Sitzungsb. niederrh. Gesellsch. Natur. n. Heilk., 

 IS'-S, 298 (1885). 



Among the remains of animals in my hands found in the ancient lake 

 basin of Florissant are about forty specimens of an onisciform arthropod, 

 about a centimeter in length, whose affinities have proved very perplexing. 

 This does not result from poorness of preservation, for among the numerous 

 specimens apparently all the prominent external features are found com- 

 pletely preserved, and even the course of some of the internal organs may 

 occasionalh' be traced ; but it presents such anomalies of structure that we 

 are at a loss where to look for its nearest kin. 



It appears to be an aquatic animal. Its body consists of three large 

 subequal thoracic joints, and an abdomen about half as large again as any 

 one of them, with occasional indications of a feeble division into four seg- 

 ments. These are the only jointed divisions that can be found in the body, 

 there being no distinct head. The thoracic segments are so considered 

 because each bears a pair of legs, which occur nowhere else. Their dorsal 

 plates are large, flat longitudinally, and arched transversely, smooth, and 

 deeply and narrowly notched in the middle of the front margin. The first 

 plate, in which the median notch is more conspicuous and open than in the 



' Since tbis was written Bri)ngniart has described a speoics from the Carboniferons deposits of 

 ('(iiiimeutrv, France. 



